The invention relates to a method for controlling an automatic transmission, and to an automatic transmission which is controlled in this way.
Motor vehicles having an internal combustion engine and an automatic transmission having a torque converter consume an unnecessarily large amount of fuel when at a standstill with a gear stage selected, for example in shift position “D”, since the internal combustion engine works via the torque converter against the stationary converter turbine. Furthermore, the operating state, in particular in diesel engines, can lead to unpleasant vibration excitations and noises in the drivetrain.
DE 19912480 A1 discloses an automatic transmission having three single planetary gear sets and having hydrodynamic torque converters, which automatic transmission has a “braked neutral” position. In a “braked neutral” position, the sun gear and the planet carrier of the first planetary gear set are fixed in a non-rotatable manner relative to the housing by means of two brake clutches as a result of simultaneous actuation of the first and second brake clutches, as a result of which the drive output shaft which is directly connected to the ring gear of the first planetary gear set is likewise locked. Furthermore, a third brake clutch which is connected to the third planetary gear set is opened. In this way, no torque can be transmitted between the input shaft and output shaft, and the input shaft can rotate freely. The “braked neutral” position is automatically selected by the transmission controller when the vehicle comes to a standstill with a gear stage (D, 1, 2, 3, 4 etc.) selected.
However, the complete opening of the third brake clutch leads to unacceptable shocks during starting, since the third brake clutch must first be placed quickly into the contact position. Complete prevention of a jerk when departing from the “braked neutral” position shortly before starting is barely possible, in particular if the throttle pedal is depressed quickly after the footbrake is released. This is linked to the effect of the vehicle being driven as a result of the braking of the turbine inertial mass. Since the footbrake has already been released, the vehicle experiences a slight acceleration which ends once the turbine has come to a standstill. The faster the turbine is braked and the faster the vehicle is thereby ready for starting, the more clearly perceptible the jerk will be.